Because of the complicated interaction of these stats, and the way they multiply by weapon damage, it's difficult to say that one is 'better' than the other. It depends mostly on the amount of each you have. If the two come out to a wash, keep in mind that increasing attack speed has the ancillary benefit of improving your mobility in between casts, while increasing your stats has some survivability benefits.

Increasing attack speed also means more chances for crits, which is part of the reason that your Damage seems to increase more than it should. The one case where it doesn't help is with Damage over Time spells. It will make them "tick" faster, but it won't actually increase the amount of damage they do.
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bwarnerJun 19 '12 at 13:20

Your attack speed will increase the amount of attacks you put out per second, obviously. But what this really means is that it scales your base damage, which your int is directly related to. So as your base damage increases, the increased damage from your attack speed bonus will also increase significantly. So reaching that crucial point of when to focus on attack speed is kinda up to your discretion, unless you really feel like running the numbers and finding the exact point at which the benefit of switching from focusing on int to attack speed is.

Just a rough guess but I'm guessing you want to start looking for attack speed items once you get around 10-20k dmg, at least that's what the mage and demon hunter I run with did. Once you do reach "the tipping point" attack speed makes a huge difference though. My demon hunter buddy is still using level 36 gloves with int and 12% attack speed, even 100+ dexterity wont make up the difference...

For us wizards IAS will not pump up the damage done by our spells. It just lets us cast them faster. In other words all that extra damage you see on the stat screen due to IAS bonuses doesn't actually do anything to make our individual spells hit any harder.

If you have two build with the same exact DPS but one relies heavily on IAS gear and the other doesn't, the non-IAS build's spells will do much higher damage per cast. Because the IAS build lets you cast faster it will have the same damage output over time as the first build but you will have to burn way more AP to do it unless you are using signature spells.

The one real advantage to the IAS build is that you will trigger "on hit" effects far more often since the spells are being cast more rapidly but hit less hard. This is great for critical mass and all sorts of other fun things like life on hit etc. but you have to generate tons of AP to keep it going.

There is also one spell that breaks the rules for IAS damage scaling which is energy twister. Unlike every other spell in the wizard's arsenal, the damage tick rate does scale up with IAS on the twisters and it makes it a much nastier spell with a good attack speed.

A wizard is different from a demon hunter. I have no idea about demon hunters on this topic. IAS does not increase the damage number you see above the heads of your targets. In fact, it lowers unless it is supplemented with the same amount of actual damage "like 25-50 on a ring" but you will have faster attacks. The one thing I tested though is to see the actual stat weights of the two and I found that by dropping all of my IAS for "damage" my numbers were actually bigger by a much bigger margin than how fast my attacks were if i were to leave my IAS on.

For example I swapped out a 1 hand weapon and a source for a 2 hand hammer, also a ring, the hammer has 1107 damage on it, the 1 hand weapon has 700ish and a source with about 300 damage, this should be about the same damage in theory but because the 1 hand is faster it actually increases the "damage" stat by about 1.5k, that's just the weapons. Now the rings I swapped out were 8-16 damage, 14% IAS for 25-50 damage and 45% crit damage.

The ring with IAS is according to the damage stat a 1.3k damage increase, so altogether it's almost 3k damage gain with IAS according to the damage stat. However my attack speed when I take off all the IAS gear and go for straight damage stats only drops by .5 and I lose about 3k damage as a stat, but when I attack I'm seeing my hits go from 40k-80k hits vs the 20k-40k hits with IAS. Now granted they do hit slightly faster, but I just don't feel that the damage is all that comparable. Hope this helps, sorry for the non exact numbers but I saw your post during maintenance so I can't quote directly from the game.